This sequence of images, taken by the Hubble Space Telescope's Wide Field Camera 3, shows a moon circling the dwarf planet 2007 OR10. The top four images were taken Nov. 6, 2009; the bottom four, on Sept. 18, 2010.

As 2007 OR10 orbits the sun, it changes position with respect to the background stars (the bright objects that change from panel to panel). The snapshots reveal that the moon moves with the dwarf planet, proving that it is gravitationally bound to the larger body.

2007 OR10 is the third-largest known dwarf planet, behind Pluto and Eris, and the largest unnamed world in the solar system. The pair is located in the Kuiper Belt, a realm of icy debris left over from the solar system's formation.

The dwarf planet is about 950 miles across; the moon is estimated to be 150 miles to 250 miles in diameter. 2007 OR10, like Pluto, follows an eccentric orbit, but it is currently three times farther than Pluto is from the sun.